1. Field of the Invention
The invention relates to a heat-sealable, shrinkable, multilayer polypropylene film which comprises at least one base layer containing a propylene polymer or a propylene polymer mixture and at least one outer layer which comprises a mixture or a blend of two polymer components.
The film according to the invention is distinguished by a characteristic matte surface or appearance and high shrinkage in the longitudinal and transverse directions.
The invention further relates to a process for the production of the multilayer film and to the use of the film.
2. Description of Related Art
Shrink films are known in the art and are widely used as packaging films.
Shrink packaging is usually produced starting from a pouch, into which the good are introduced, followed by separation/welding and subsequent shrinkage in a shrink oven. In this way, very tight, fold-free film packaging is obtained, as is particularly desirable in many cases. In order for a tight wrap to be reliably achieved, the films used must have very high shrinkage values, because the film after separation/welding surrounds the contents relatively loosely. This is due to the fact that the welding wire cannot be moved right up against the packaged goods. High shrinkage values of the films are therefore a prerequisite for their use as conventional shrink packaging. Such films are described, for example, in EP-A-0 436 196, DE-A-19 54 467, DE-A-32 15 120 and EP-A-0 350 859.
Heat-sealable films are likewise widespread and generally known as packaging materials. The heat-sealability of the material enables the packaging of products on high-speed packaging machines, which allows considerable economic advantages to be achieved.
Heat-sealable, biaxially oriented, multilayer polyolefin films in which the base layer comprises propylene homopolymers and the heat-sealing layer(s) (outer layer(s) or top layer(s)) comprises heat-sealable olefin polymers are described in numerous publications (for example EP-A-0 194 588, EP-A-0 008 904 and U.S. Pat. No. 4,419,411). The multilayer polyolefin films have important properties for packaging films, such as a large heat-sealing range, good heat-sealability and low haze and high sheen.
The heat-sealable films described are generally produced in such a way that they are as dimensionally stable as possible, i.e. these films have the lowest possible shrinkage in the longitudinal and transverse directions. In general, packaging films of this type exhibit shrinkage values at 120.degree. C. of less than 5% in the longitudinal direction and less than 4% in the transverse direction. The prevailing opinion hitherto was that films having these low shrinkage values give a visually good wrapping appearance. Contrary to this opinion, the packages are wrapped in a loose or bloused manner and have folds and billows at the edges and are therefore unsuitable for products in which particularly tight packaging is required.
In the search for heat-sealable shrink films, it has been found that conventional shrink films are unsuitable for a heat-sealable shrink film. Although coating known highly shrinkable films with a heat-sealable layer enables them to be used in packaging machines, the pronounced shrink properties, which ensure the desired tightness of the packaging, also cause the seal seams to shrink so much that the visual appearance of the packaging is adversely changed. In addition, all conventional, known shrink films are unsatisfactory with respect to their passage through the machine and their antistatic properties.
EP-A-0 489 373 discloses heat-sealable shrink films built up from a polypropylene (PP) homopolymer base layer and ethylene-propylene copolymer outer layers and having high gloss and low haze. These films have increased shrinkage compared with conventional heat-sealable packaging films, but the shrink properties are inadequate if tight coverings are required.
German Patent Application P 41 35 096.0 discloses multilayer polyolefin films comprising a polypropylene base layer and an outer layer made from an HDPE blend. This HDPE blend comprises HDPE and copolymers and/or terpolymers made from .alpha.-olefins and, if desired, polypropylene. The outer layer of the multilayer film has minimum sheen and maximum haze, giving the film a characteristic matte appearance.
U.S. Pat. No. 4,578,316 discloses a biaxially oriented multilayer polyolefin film whose base layer comprises a polypropylene homopolymer and whose outer layer comprises a blend of polypropylene and MDPE and/or HDPE. This polyolefin film has a low coefficient of friction together with a readily wettable surface. However, this film also has excellent optical transparency.
WO 89/10839 discloses multilayer polyolefin films whose base layer comprises a propylene polymer and whose outer layers comprise either HDPE or an ethylenepropylene copolymer or a polypropylene. The film is stretched under particular temperature conditions. The film has a low coefficient of friction and good wettability, but simultaneously also has high sheen and low haze.
EP-A-0 367 613 discloses a multilayer film comprising a vacuole-containing polypropylene base layer and an inscribable outer layer which contains a first polymer having a melt flow index of .ltoreq.1 g/10 min and a second polymer which is incompatible with said first polymer. Incompatible being defined as the second polymer being present as a separate phase. An example given of the first polymer is an HDPE having a density from 0.92 to 0.97 g/cm.sup.3. The incompatible polymer is a polypropylene or a copolymer of propylene or a terpolymer of propylene. The film described has an opaque appearance, i.e. it is essentially non-transparent to light. The film described has a density of 0.69 g/cm.sup.3. This film cannot be employed for many applications due to its opacity.